My 2000 JXI w/ 165k mi. just started running on 5 cylinders intermittently. Codes present: P0203 Injector circuit malfunction- cyl 3, P0303 (pending) Cyl #3 Misfire Detected.

Symptoms during driving yesterday:
a) Yesterday- cold start- runs smooth on all cyl.
b) after 30 min drive, turn off, restart in 20 min, running rough, seemingly on 5 cyl.
c) during next 45 min (frwy) drive, it seemed to be running on 6cyl later part of drive, but not sure because harder to tell at frwy speeds.
d) cleared codes- 60 min drive back home (don't recall if missing or not?)
e) Today- cold start- runs smooth on all cyl. re-read codes: same as above (P0303 '#3 misfire detected' still 'pending')

Questions:
1) Any chance fuel injector cleaner could help this issue?
2) Is #3 the middle cyl in back under intake plenum / ?
3) Any hope I can avoid pulling the intake plenum to access this injector for testing or replacing?
4) Why might the issue be intermittent, but somewhat predictably so?

Thanks for any insights.

 

Do you stil have the car?